1. Field of the Invention
The present invention relates techniques for assaying nuclear materials, and more specifically, it relates to Gamma- and neutron-spectroscopy.
2. Description of Related Art
Gamma- and neutron-spectroscopy are powerful techniques widely used to determine the composition of nuclear materials non-destructively. Gamma-spectroscopy relates the measured intensity of characteristic gamma-rays from radioactive decay to the concentration and the ratio of gamma-emitting isotopes in the sample. The precision of isotope ratio measurements typically increases when based on intense gamma-lines with similar energies, because variations in detection efficiency and matrix effects are reduced. High-purity Ge detectors with (moderately) high energy resolution are therefore used widely, and several analysis routines (such as Multi-Group Analysis, MGA) have been developed for non-destructive evaluation of isotope ratios with Ge detectors.
Neutron-spectroscopy is currently not as common as Gamma-spectroscopy, primarily because high-resolution neutron spectrometers are either very large and have low detection efficiency (e.g., time-of-flight spectrometers), or have complicated response functions (e.g., 3He-based systems). Neutron spectroscopy offers the advantage to also detect non-radioactive substances embedded in a nuclear matrix, to detect nuclear materials through centimeters of shielding, and to identify the shielding material. This information can be extracted from characteristic features in the neutron spectra related to nuclear scattering resonances, and high energy-resolution detectors are required to detect the narrow resonances in the ˜MeV range.
Cryogenic detectors are a novel class of sensor technologies operating at temperatures below ˜1 K that are currently being developed by several institutions for high-resolution spectroscopy, mostly focusing on X-ray analysis. The Advanced Detector Group at LLNL is developing cryogenic detectors for high-resolution Gamma- and neutron spectroscopy. Earlier work was based on different sensor technologies such as superconducting tunnel junctions (Netel et al.), gallium- or neutron-transmutation-doped germanium (Marcillac et al., Silver et al.) or silicon thermistors (Bleile, Egelhof et al.). These technologies have so far not met the requirements for high energy resolution, efficiency and count rate required for sensitive nuclear analysis. For detector designs based on tunnel junctions the charge transport from the absorber to the sensor was to inefficient, and for semiconductor thermistor technologies either energy resolution or efficiency or maximum count rates are too low.